Sand moisture detector including a plow nose housing and roller electrode

ABSTRACT

Apparatus is provided for use in a moisture control system which responds to the conductivity of foundry sand disposed between two locations along a reach of a conveyor belt carrying a quantity of foundry sand. At each location there is a device which includes a plow for shaping the sand into a layer or swath of uniform thickness and a rotary electrode which rides in the layer, rotating with the movement of the layer past it. Rotation is achieved either by driving the rotary electrode by an electric motor, or by having the electrode ride on the reach and be driven by the passing belt. Scraper blades keep the exposed surfaces of the rotary electrodes free of adhering sand. The electrodes are mounted at a station which provides means for supporting the electrodes and moving them into position.

Unite States Pate Nutter Feb. 29, 1972 [72] Inventor: Orville C. Nutter,314 lngalton, West Chicago, Ill. 60185 [22] Filed: Sept. 21, 1970 21Appl. No.: 73,709

Primary ExaminerEdward E. Kubasiewicz Attorney-Silverman & Cass [57]ABSTRACT Apparatus is provided for use in a moisture control systemwhich responds to the conductivity of foundry sand disposed between twolocations along a reach of a conveyor belt carrying a quantity offoundry sand. At each location there is a device which includes a plowfor shaping the sand into a layer or swath of uniform thickness and arotary electrode which rides in the layer, rotating with the movement ofthe layer past it. Rotation is achieved either by driving the rotaryelectrode by an electric motor, or by having the electrode ride on thereach and be driven by the passing belt. Scraper blades keep the exposedsurfaces of the rotary electrodes free of adhering sand. The electrodesare mounted at a station which provides means for supporting theelectrodes and moving them into position.

19 Claims, 9 Drawing Figures PATENTEDFEBZS I972 3,646,437

SHEET 3 OF 3 -Z 170 138 122 2&6 %g

SAND MOISTURE DETECTOR INCLUDING A PLOW NOSE HOUSING AND ROLLERELECTRODE BACKGROUND OF THE INVENTION This invention finds particularutility in the art of conditioning foundry sand mixtures.

In foundries which are required to make castings on a production basisthe sand mixture is used repeatedly and must be conditioned to enablesuch repetitive use. The finally used mixture must have uniform bulkdensity and must be of proper temperature and moisture content.Apparatus is known which mixes and mulls the sand and which cools it.Fresh sand and other ingredients may be added in the process ofconditioning the sand.

In the process of conditioning sand water is generally added tomaterials having a variable amount of moisture to reach a preestablishedpercentage with as close an accuracy as possible to maintain consistentdesirable molding properties of the material which is adversely affectedby deviations of as little as plus or minus 0.25 percent.

One of the principal problems has been the difficulty of continuouslyand accurately sensing the moisture content of the material in order todetermine the proper amount of moisture to be added to reach therequired percentage after a final mixing.

In the prior art, the resistance between two points along a moving layerof sand is a part of an electrical circuit to indicate moisture contentand/or to control the water addition required.

A Wheatstone bridge circuit has the sand resistance in one of its armsand a manually adjusted or preset control resistance between two of thearms provides a signal to control the moisture adding apparatus. Themanual adjustment is a control potentiometer that in turn positions thedegree of opening of a valve. When the bridge is balanced, a set amountof moisture is continuously added to give the desired percentage. Thewater will be sprayed on the moving mass of sand either before or afterthe station where the sensing is taking place. This is well known in theprior art.

In order to sense the moisture content of the moving mass of sand, aswath having a uniform layer is plowed at the measuring station, and twoprobes are provided spaced along the path of movement of the layer onthe belt. These probes are electrodes that extend into the layer of sandso that they intercept the moving layer. Since foundry sand is amixture, including various types of binders, the sand is sticky, evenafter it has been fluffed and mixed. Accordingly, in the past, the sandmixture adheres to the probes of the prior art, and builds up on theseprobes, thereby changing their electrical characteristics. For example,it is not unusual for the resistance between a pair of probes at thestart of a run to increase several times after a few minutes of use,irrespective of whether there has been a change in the moisture contentof the sand mixture. This buildup of a sand mass on one or both of theprobes introduces such error into the measurement that the informationacquired by the apparatus becomes practically useless. The alternativeto such problem is to continuously clean the probes, a process whichfrustrates the desired benefit of automatic operation.

The invention herein solves the problem and provides a reliable andrugged apparatus for use in a moisture control circuit for foundry sandmixtures.

SUMMARY OF THE INVENTION According to the invention, in place of theprior art probes, the measuring station is provided with a pair ofrotary electrodes that are in the form of wheels engaging into theuniform layer of sand provided by plows which are combined with therespective electrodes. The wheels are either mechanically driven bysuitable motors or engage against the conveyor belt and are frictionallydriven by the passing of the belt. Scraper blades keep the wheels cleanof adhering sand. The electrical contact to each of the rotaryelectrodes is picked up by connecting a conductor to the metal bushingin which the shaft of the wheel is joumaled. This provides a wipingcontact between shaft and bushing. The rotary electrode is mounted in ahousing which is insulated, the housing in turn being supported inswinging condition from a suitable framework at the measuring station.The wheels of a pair of electrodes are staggered along the length of themoving layer of sand so that the follower electrode engages into a massof sand that is of uniform thickness and is undisturbed by the passageof the leader electrode. Each electrode has a plow for levelling thesand layer prior to engagement of the electrode therein.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view of acombined rotary electrode and plow with portions broken away to show theconstructional details;

FIG. 2 is a top plan view of the combined electrode and plow of FIG. 1taken generally on the line 22 and in the direction indicated;

FIG. 3 is a fragmentary sectional view taken generally along the line3-3 of FIG. 1 and in the direction indicated;

FIG. 4 is a sectional view taken generally along the line 4-4 of FIG. 2and in the direction indicated;

FIG. 5 is a fragmentary sectional view taken generally along the line5-5 of FIG. I and in the direction indicated;

FIG. 6 is a perspective view of a measuring station using a pair ofrotating electrodes, the same being shown with respect to the upperreach of the moving conveyor belt;

FIG. 7 is a side elevational view of a similar but slightly modifiedinstallation;

FIG. 8 is a diagrammatic view showing the manner in which the inventionis used on a moving mass of sand; and

FIG. 9 is an end elevational view of the structure shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Generally, the inventioncomprises rotary electrodes which are kept clean so that resistancemeasurements made by use of these electrodes will be accurate and notaffected by any sand buildup. The electrodes are combined with plowsthat produce uniform thickness layers or swaths of sand into which theelectrodes bite. There are at least two electrodes for each installationtogether with two plows. Electrodes are staggered with respect to theline of movement of the conveyor belt over which the electrodes aredisposed so that the rear or followerelectrode will not engage in anygrooves produced by the leader electrode. The rotary electrodes may bedriven by motors independent of the movement of the belt or they may bedriven by the belt itself. In some installations, one electrode may bemotor-driven and the other may be friction-driven.

Looking now at FIGS. 1 through 5, there is illustrated a simplified formof a combined electrode and plow designated generally by the referencecharacter 20. A housing 22 is provided for a metal wheel 24 which issuitably mounted for rotation with a substantial portion protruding fromthe bottom of the housing.

The housing of the structure 20 is formed of a central plate 26 ofinsulating material sandwiched between the plates 28 and 30, also ofinsulating material. Each of the plates 28 and 30 has a cylindricalbronze bushing mounted therein as shown at 32 and 34 in FIG. 5. Thecentral plate 26 is provided with an arcuate recess 36 which is formedby cutting away the plate. The front end of the housing 22 has a greatervertical dimension than the rear end of the housing 22. A pair of blocks38 and 40 are engaged over the outer plate 28 and 30 respectively toprotect and cover the bushings 32 and 34 respectively, these passingcompletely through the respective plates 28 and 30. The blocks 38 and 40are held in place by suitable screws such as 42 screw-threaded into therespective blocks 28 and 30. The plates 28, 26, and 30 are clampedtogether by means of suitable bolts 44 and 46 which pass through thesame and are held in place by nuts 48 and 50 respectively.

A steel wheel 24 has a stub shaft 54 affixed thereto, the stub shaftbeing journaled at its ends in the bushings 32 and 34 respectively. Apair of annular spacers 56 center the wheel within the recess 36. Themetallic wheel 24 makes a wiping electrical contact with the bushings 32and 34, and a screw 58 passing through the corner 68 of one of theplates 28 and 30 is screw-threaded into the bushing 34 as shown inFIG. 1. An electrical lead 70 connects the screw 58 into the bridgecircuit of the moisture-supplying device. Such bridge circuit is wellknown and hence need not be illustrated.

The right-hand end of the apparatus 20 of FIGS. 1, 2, and 4 is theleading end and hence the back end is at the left. As seen, asubstantial portion of the wheel 24 protrudes at the bottom of thehousing 22. The forward end of the housing is tapered to form the plownose 72 that is intended to part oncoming sand while forming a swathhaving a uniform thickness. At the back end of the device 20, there isprovided a pair of scraper blades 74 and 76 mounted for scraping thewheel 24 just before the periphery of the wheel passes into the housing22. These scrapers are formed of some insulating material such as anelastomer and are clamped between the clamping plates 78 and 811 and thebrackets 82 and 34 respectively. These brackets are in the form ofstrips of metal secured to the surface of the respective plates 28 and30 by means of the screws 86 with an intervening layer of insulatingmaterial such as an elastomer. The intervening elastomer layers aredesignated 88 and 90 and are best shown in FIG. 3.

An edge scraper 92 is mounted as shown in FIG. 1 to prevent buildup ofsand on the edge of the wheel 24. The edge scraper 92 is secured to thebottom of the housing 22 at its narrower trailing end by means of thescrews 94.

An elongate support plate 96 has depending ears 98 and 100 that areengaged by the bolts 46 and 44 respectively. At its front end, thesupport member 96 is provided with an eye 102 to enable the device 20 tobe mounted in swinging condition on a suitable support structure. Inthis way, the weight of the device 20 would keep the wheel 24 riding onthe reach of a conveyor belt causing the same to rotate as the beltmoves by. Also, the device 20 could be swung up out of the way about thecar 102 when it is not in use.

In order to prevent the sand from building up on the device itself, theplow nose 72 and most of the housing 22 is covered by a sheet ofsynthetic resinous material, such as tetrafluoroethylene. As seen, theprotective sheet 104 covers the plow nose 72, the blocks 38 and 40, andthe brackets 82 and 84. Any suitable means for holding the protectivesheet 104 in place may be used, such as screws 106 engaging into variousparts of the housing 22. An additional shaped sheet of this samematerial may be secured to the bottom of the device 20 as shown at 108.The purpose of the bottom sheet is to prevent sand from adhering to thebottom of the housing 22.

In use, the device 20 will ride on a belt 110, the frictional engagementbetween the belt and the wheel 24 causing the wheel to rotate in thedirection indicated by the arrow of FIG. 1 as the belt moves to the leftrelative to the device 20. The plow nose 72 will divide the sand andprovide a layer or swath of uniform thickness which is cut by the wheel24.

The device 20 is driven by the belt 110, in which case, the wheel 24 ispennitted to contact the belt. By slight modification, it is feasible tohave the wheel 24 driven by an electric motor or the like, in which casethe wheel 24 will be spaced above the belt 1 slightly.

In use, there will be two electrodes," that is, two of the devicessimilar to but with the wheels staggered so that the trailing orfollower wheel will not engage in a groove produced by the leader orforward wheel.

In FIG. 6, there is illustrated a belt 110 moving on a bed 112 to theleft. This is the upper reach of the belt, the lower reach not beingshown in this view but it is understood that the lower reach will bemoving in the opposite direction. At the measuring station, there is aframework 114 supported by suitable standards 116, the frameworkcarrying two devices, 120 and 122. Each device has some form of supportstructure 124 and 126 respectively, such structure being the equivalentof the support plate 96 of FIG. 1. The front end of each of the devicesand 122 is pivoted at 128 and 130 respectively, and, in this case, eachsupport structure has a motor mounted thereon. The motor 132 is intendedfor driving the rotary wheel 134 of the device 120, and the motor 136 isintended to drive the rotary wheel 138 of the device 122. The rear endsof the respective structures 124 and 126 engage suitable transversesupport members such as shown at 140 and 142, such engagement beingadjustable in order to adjust the spacing of the respective wheels fromthe belt 110. If desired the entire devices 120 or 122 may be swung uponthe respective pivots 128 and 130 out of use. In each case, there is abelt 144 and 146 driven by the respective motors 132 and 136, thesebelts being engaged over suitable sheaves (not shown) secured to theshafts of the respective lheels 134 and 138.

In FIG. 7, a modified form of the structure of FIG. 6 is shown, suchmodification consisting of remoing the belt 144 and lowering the frontdevice 120 so that the front device 120 has its wheel 134 engaging thebelt 110 and is frictionally driven by the belt. As will be seen, thetrailing device 122 has its wheel 138 slightly spaced above the belt I10to permit the motor 136 to drive the wheel 138. In FIG. 6, if the device120 is connected to be driven by the motor 132, its wheel 134 will alsobe spaced above the belt 110, but, in FIG. 7, the belt 144 has beenremoved and the wheel 134 is therefore lowered to come into frictionalengagement with the belt 110 so that it is driven by the belt 110 duringthe movement.

A simple mechanism is shown at 143 and in FIG. 7 for adjusting thepositions of the respective support structures 124 and 126 above thebelt 110.

Looking now at the diagram of FIG. 8, it can be seen that the belt 110has a mass of sand 150 moving to the left as viewed in FIG. 8 supportedon the belt. The sand is the mixture used for forming molds and ahead ofthe measuring apparatus it may have masses of nonuniform thicknesses onthe belt. The device 120 includes a plow nose 152 and apair of thewheels 134, which are either driven by the electric motor 132 or drivenbecause of frictional engagement with the belt 1 10. The wheels 134 aremounted on a shaft 154, and, for purposes of driving the same by amotor, a sheave is shown at 156. Scraper blades designated are providedto keep each of the wheels 134 clean. Note that two pair of such bladesare provided where the device has a pair of electrode wheels.

The plow nose 152 produces a swath of uniform thickness of sand, asindicated at 162. Each of the wheels 134 therefore will always engageinto a uniform layer of sand. The trails or grooves caused by the wheels134 are shown at 164. The follower or trailing device 122 also has aplow nose 166 which maintains the swath of substantially uniformthickness on the belt 1 10, this swath or strip being shown at 168. Thetrailing or follower wheel 138 thus engages in a layer of sand 168 whichis of uniform thickness. It also has the scrapers 170 to keep it clean.

The belt returns in a lower reach 111 as shown in FIGS. 7 and 9.

It will be seen that the wheels 134 and 138 are staggered with respectto one another along the length of belt 1 10 so that in the event thegrooves 164 are not smoothed and filled in by the plow 166, there willstill be a clear track for the wheel 138.

In FIG. 7 there is illustrated an electrical lead 172 which connects tothe bearing of the wheel 134 by way of a suitable screw 174, and theelectrical lead 176, which connects through the screw 178 to the bearingof the wheel 138.

There will be a thermistor engaging into the sand 150 in the vicinity ofthe measuring station illustrated in FIGS. 6 through 9. This is notshown in the drawings since it forms no part of the invention.

As previously indicated there is a wheatstone bridge for controlling theamount of water that will be added to the sand. One balance leg of thebridge is the sand which is subtended between the electrode wheels ofthe apparatus, such as for example between the wheels 134 and 138. Theother comtntnm: and

ponents of the bridge are adjusted manually so that a balanced conditionwill not energize the water-adding apparatus. When an unbalance occurs,as occasioned by an increase in the resistance between the rotaryelectrodes, water will be added. The conductivity of sand decreases whenthe moisture content decreases. So long as the electrodes are keptclean,the resistance measured between electrodes will be a substantially truefunction of the moisture content of the swath of sand and hence willprovide reliable and reproducible information,

Various modifications may be made without departing from the scope ofthe invention as defined in the appended claims.

What it is desired to secure by Letters Patent of the United States is:

1. An apparatus for measuring the moisture content of pulverulentmaterial passing along the reach of a conveyor belt and which includes ameasuring station having a pair of electrodes spaced from one anotheralong the length of the belt and adapted to engage the passing mass ofmaterial electrically to contact the same for responding to theconductivity of the material between the electrodes, the electrodes eachcomprising a wheel of conductive material arranged to rotate in avertical plane normal to the belt reach, and parallel with the line ofmovement thereof, each electrode having means for supporting the sameover the reach and being insulated from said support means, housingscarrying the respective electrodes and each having a forwardly directedplow nose spaced above the reach whereby to form a swath of pulverulentmaterial of uniform thickness as the mass passes, each wheel beingspaced rearwardly of its plow nose and arranged to engage into a swath,means for connecting an electrical lead to each wheel in a wipingcontact, means adjacent the rear edge of each wheel for scraping off anymaterial picked up by the wheel while rotating, and means for rotatingeach wheel while the belt is moving past the same.

2. The structure as claimed in claim 1 in which said lastmentioned meanscomprise the belt reach, one of said wheels being frictionally engagedto the belt reach on its bottom edge.

3. The structure as claimed in claim 1 in which said lastmentioned meanscomprise an independent source of rotation carried on one of saidsupporting means and mechanically connected to a wheel, the bottom edgeof the wheel being slightly spaced above the belt reach.

4. The structure as claimed in claim 1 in which one of said means forrotating comprises an independent source of rotation carried on saidsupporting means and mechanically connected to one of said wheels withthe bottom edge of said one wheel spaced above the belt reach, and thesecond of said means for rotating comprises the belt reach, the secondof said wheels having its bottom edge in engagement with the belt reach.

5. The structure as claimed in claim 1 in which the wheels lie in planeswhich are parallel to and spaced from one another whereby the electrodesare staggered along the belt reach.

6. The structure as claimed in claim 1 in which each supporting means ispivotally mounted to enable the swinging of the electrode housing tocarry its electrode into or out of the path of the moving mass ofpulverulent material.

7. The structure as claimed in claim 1 in which at least the plow noseis covered by a resinous member that resists adherence of saidpulverulent material thereto.

8. The structure as claimed in claim 1 in which said lastmentioned meanscomprise an independent source of rotation carried on the supportingmeans of each of said electrodes and being respectively connected tosaid wheels, the bottom edges of the said wheels being slightly spacedabove the belt reach.

9. The structure as claimed in claim 2 in which the supporting meansincludes a pivotal connection at one end of the housing with theremainder of the housing being free to swing downwardly whereby theweight of the housing and electrode carries the same into saidfrictional engagement with the belt reach.

10. The structure as claimed in claim 1 in which one wheel is mounted inits housing as a single wheel and the other wheel has a tlurd wheelcoaxial therewith m its housing whereby to form a pair of spaced wheels,the plane of the single wheel being parallel with and disposed betweenthe planes of the pair of wheels.

11. The structure as claimed in claim 10 in which the housing with thepair of wheels is forward of the housing with the single wheel wherebythe single wheel will not engage any grooves formed in the swathproduced by the pair of wheels.

12. The structure as claimed in claim 1 in which said scraping meansinclude blades juxtaposed relative to opposite faces of the wheel.

13. The structure as claimed in claim 1 in which said scraping meansinclude blades juxtaposed relative to opposite faces of the wheel and ascraping member arranged to remove pulverulent material from the edge ofthe wheel.

14. For use with apparatus for measuring the moisture content ofpulverulent material passing along the reach of a conveyor belt, anelectrode adapted to engage in said pulverulent material and besupported over the belt to make electrical contact with said material,said electrode comprising:

a housing of insulating material having a recess in the center thereofand a plow nose, a wheel of conducting material joumaled in said recessand having a substantial portion thereof protruding from the bottom ofsaid housing, means for making an electrical wiping contact with saidwheel and means at one edge of said wheel arranged to scrape pulverulentmaterial from the exposed portions of said wheel as the wheel isrotated.

15. The electrode of claim 14 in which there is a covering at least overthe plow nose which is made of a resinous material to which pulverulentmaterial will not adhere.

16. The electrode of claim 14 in which the means for making contactcomprise bushings of conductive materials in said housing, said wheelbeing journaled in said bushings, and there being a conductive memberextending from one of said bushings to the exterior of said housing.

17. The electrode of claim 14 in which the scraping means comprise apair of opposite blades engaging the exposed faces of the wheel adjacentone edge thereof opposite said plow nose.

18. The electrode of claim 17 in which said blades are formed of anelastomeric material.

19. The electrode of claim 17 in which there is an additional scraperslightly spaced from the said one edge for preventing buildup ofpulverulent material on the edge of said wheel.

1. An apparatus for measuring the moisture content of pulverulentmaterial passing along the reach of a conveyor belt and which includes ameasuring station having a pair of electrodes spaced from one anotheralong the length of the belt and adapted to engage the passing mass ofmaterial electrically to contact the same for responding to theconductivity of the material between the electrodes, the electrodes eachcomprising a wheel of conductive material arranged to rotate in avertical plane normal to the belt reach, and parallel with the line ofmovement thereof, each electrode having means for supporting the sameover the reach and being insulated from said support means, housingscarrying the respective electrodes and each having a forwardly directedplow nose spaced above the reach whereby to form a swath of pulverulentmaterial of uniform thickness as the mass passes, each wheel beingspaced rearwardly of its plow nose and arranged to engage into a swath,means for connecting an electrical lead to each wheel in a wipingcontact, means adjacent the rear edge of each wheel for scraping off anymaterial picked up by the wheel while rotating, and means for rotatingeach wheel while the belt is moving past the same.
 2. The structure asclaimed in claim 1 in which said last-mentioned means comprise the beltreach, one of said wheels being frictionally engaged to the belt reachon its bottom edge.
 3. The structure as claimed in claim 1 in which saidlast-mentioned means comprise an Independent source of rotation carriedon one of said supporting means and mechanically connected to a wheel,the bottom edge of the wheel being slightly spaced above the belt reach.4. The structure as claimed in claim 1 in which one of said means forrotating comprises an independent source of rotation carried on saidsupporting means and mechanically connected to one of said wheels withthe bottom edge of said one wheel spaced above the belt reach, and thesecond of said means for rotating comprises the belt reach, the secondof said wheels having its bottom edge in engagement with the belt reach.5. The structure as claimed in claim 1 in which the wheels lie in planeswhich are parallel to and spaced from one another whereby the electrodesare staggered along the belt reach.
 6. The structure as claimed in claim1 in which each supporting means is pivotally mounted to enable theswinging of the electrode housing to carry its electrode into or out ofthe path of the moving mass of pulverulent material.
 7. The structure asclaimed in claim 1 in which at least the plow nose is covered by aresinous member that resists adherence of said pulverulent materialthereto.
 8. The structure as claimed in claim 1 in which saidlast-mentioned means comprise an independent source of rotation carriedon the supporting means of each of said electrodes and beingrespectively connected to said wheels, the bottom edges of the saidwheels being slightly spaced above the belt reach.
 9. The structure asclaimed in claim 2 in which the supporting means includes a pivotalconnection at one end of the housing with the remainder of the housingbeing free to swing downwardly whereby the weight of the housing andelectrode carries the same into said frictional engagement with the beltreach.
 10. The structure as claimed in claim 1 in which one wheel ismounted in its housing as a single wheel and the other wheel has a thirdwheel coaxial therewith in its housing whereby to form a pair of spacedwheels, the plane of the single wheel being parallel with and disposedbetween the planes of the pair of wheels.
 11. The structure as claimedin claim 10 in which the housing with the pair of wheels is forward ofthe housing with the single wheel whereby the single wheel will notengage any grooves formed in the swath produced by the pair of wheels.12. The structure as claimed in claim 1 in which said scraping meansinclude blades juxtaposed relative to opposite faces of the wheel. 13.The structure as claimed in claim 1 in which said scraping means includeblades juxtaposed relative to opposite faces of the wheel and a scrapingmember arranged to remove pulverulent material from the edge of thewheel.
 14. For use with apparatus for measuring the moisture content ofpulverulent material passing along the reach of a conveyor belt, anelectrode adapted to engage in said pulverulent material and besupported over the belt to make electrical contact with said material,said electrode comprising: a housing of insulating material having arecess in the center thereof and a plow nose, a wheel of conductingmaterial journaled in said recess and having a substantial portionthereof protruding from the bottom of said housing, means for making anelectrical wiping contact with said wheel and means at one edge of saidwheel arranged to scrape pulverulent material from the exposed portionsof said wheel as the wheel is rotated.
 15. The electrode of claim 14 inwhich there is a covering at least over the plow nose which is made of aresinous material to which pulverulent material will not adhere.
 16. Theelectrode of claim 14 in which the means for making contact comprisebushings of conductive materials in said housing, said wheel beingjournaled in said bushings, and there being a conductive memberextending from one of said bushings to the exterior of said housing. 17.The electrode of claim 14 in which the scraping means comprise a pair ofopposite blades engAging the exposed faces of the wheel adjacent oneedge thereof opposite said plow nose.
 18. The electrode of claim 17 inwhich said blades are formed of an elastomeric material.
 19. Theelectrode of claim 17 in which there is an additional scraper slightlyspaced from the said one edge for preventing buildup of pulverulentmaterial on the edge of said wheel.